The present invention relates to binder compositions for use in forming sintered parts by powder injection molding and to green compositions of the binder composition and inorganic powders, in which the binder compositions may include additional components which provide a broader range of control of reverse debinding, and to methods of using such binder compositions. The binders of the present invention require fewer steps to produce a part, have higher thixotropic energy, melt at a lower temperature, provide a green body having high strength, and decompose thermally in a clean, substantially ash-free burnout to yield simple, environmentally safe products, and may further include additives by which the rate and final temperature of debinding may be selected and controlled.
Processes for forming shaped articles from particulate mixtures are known in the art. Classically, a desired particulate material is mixed with a binder and then formed into the desired shape, this being called a green body. The green body is then fired to provide a fusion of the particulate material and to drive off the binder, thereby producing the desired shaped product with proper surface texture, strength, etc. Modern methods include press and sinter (PandS) and powder injection molding (PIM). In PandS, a mixture of one or more of a metal, metal oxide, intermetallic or ceramic powder and a small amount of binder (about 1-10%, or, on average, about 5% of the powder volume) are placed in a relatively simple mold, pressed into a green body, and then sintered. The small amount of binder is decomposed during the sintering step, so a separate step of removing the binder is not necessary. However, PandS is limited to simple parts.
In PIM, a mixture of one or more of a metal, metal oxide, intermetallic or ceramic powder and a quantity of binder from 30% to 60% of the volume of powder are heated to a liquid state and then injected under pressure into a mold to form a part. Once in the mold, the binder is removed in one or more steps and the part is fired to sinter the particles into a solid part. PIM is capable of producing quite complex parts.
In the production of shaped objects by PIM in the manner above described, it has been found that the binder, while necessary to the process, create problems. The binder must be used in order to form an object of practical use, but most of it must be removed before the part can be sintered, although in some cases a portion of the binder remains until sintering is completed.
Direct removal of the PIM binder during sintering is problematic. Many binders leave behind ash upon decomposition. When such ash combines with certain ingredients in the powder component, eutectic mixtures may be formed. Such eutectic compounds as TiC may be formed from titanium and carbon ash, and these can result in serious problems in the formed part.
Thermoplastic binders which decompose on heating have been used. However, these materials tend to soften or melt first and then decompose, creating problems on decomposition. Thermoplastic materials have been tried which decompose below their melting point and thereby remain in place until decomposition. Binders have been removed by exposure to a decomposing atmosphere, such as an acid atmosphere to decompose an acid-labile organic binder. The drawback of this approach is the use of an acid atmosphere, requiring a special chamber and hazardous material handling capabilities. Binders which are subject to catalytic decomposition also have been used, such as a polyacetal. The drawback of this approach is that the decomposition product is formaldehyde, which also requires special equipment to collect and decompose the formaldehyde.
The prior art has recognized this problem and has therefore attempted to remove the binder from the shaped green body prior to the step of sintering. Such processes have used various solvents, including organic solvents, triple-point CO2, and water to dissolve and remove the binder. While systems using such procedures can provide advantages over procedures wherein the binder is removed during firing, articles formed by removing the binder prior to firing still have the tendency to crack during the binder removal as well as during the firing operation. One reason for this is that the binder is removed from the green body by means of a solvent when the binder is in the solid state, and upon dissolution the binder, the binder-solvent mixture has a tendency to expand. This problem has been approached by various means, including heating the green body prior to exposing it to the solvent, by using a solvent to remove a portion of the binder and removing the remainder by firing, and by using a two-part binder, each part of which is soluble in a different solvent, so each solvent removes a portion of the binder, and by using the different solvents in a stepwise manner. Each of these methods includes its own drawbacks. All of these solvent-based methods suffer from the necessity of dealing with the solvents and the problems inherent therein, such as toxicity, recycling, evaporation losses and environmental considerations.
Thus, the need remains for binders which are useful, particularly in powder injection molding, which require a minimum number of steps to remove, which have high thixotropic energy, which melt at a low temperatures, which provide a green body having high strength, and which decompose thermally to yield simple, environmentally safe products, substantially free of ash, thereby yielding a binder which performs its function but which provides a process of powder injection molding which proceeds with a minimum number of process steps, can be carried out in an air atmosphere in many cases, and does not leave behind deleterious residues, either in the part or in the environment.
In addition to the foregoing needs, there exists a need for further control of the debinding process, by which the debinding time and temperature can be adjusted and controlled to match the characteristics of the inorganic material of which the green composition is comprised.
The present invention requires only simple, standard equipment which is inexpensive and commonly available. The steps of debinding and sintering may be carried out in the same equipment, on a continuous basis, thereby avoiding downtime for cooling and transfer from debinding equipment to sintering equipment.
In one embodiment, the present invention relates to a binder composition comprising a polycarbonate polymer; an ethylenebisamide wax; a guanidine wetting agent; and an additive which in use accelerates or extends debinding of the binder composition. The present invention further relates to a method for forming a part by powder injection molding, including the steps of forming a green composition comprising a binder and an inorganic powder, wherein the binder is a composition comprising a polycarbonate polymer, an ethylenebisamide wax, a guanidine wetting agent and an additive; heating the green composition to debind the green composition, wherein the additive accelerates or extends the debinding step. In one embodiment, the additive is an debinding accelerator which increases the rate of debinding of at least one of the first three elements of the binder composition. In one embodiment, the additive is an debinding accelerator which increases debinding of at least one of the first three elements of the binder composition. In one embodiment, the additive is a debinding extender which extends the time and/or increases the upper temperature of, the debinding process.
In one embodiment, the binder composition of the present invention may include both an debinding accelerator and a debinding extender. The debinding accelerator assists in quickly dispensing with the lower-temperature-debinding components of the composition, while the debinding extender extends the debinding and thereby assures that at least some of the components of the debinding composition remain to maintain the inorganic powder particles in place in the mold until the onset of sintering.
Thus, the binder composition and method of making sintered parts using the binder composition of the present invention provide the features missing from the prior art. The binder composition may be removed in a minimum number of steps, has high thixotropic energy, melts and becomes flowable at a low temperature, provides a green body having high strength, and decomposes thermally to yield simple, environmentally safe products, substantially free of ash. In addition, the additive allows increased control of the timing and temperature of the debinding steps as compared to a binder without the additive. When the additive is an debinding accelerator, the time required for debinding can be reduced. When the additive is a debinding extender, the time and/or temperature of debinding can be increased. The binder composition thereby performs its function while providing a process of powder injection molding which proceeds with a minimum number of steps, can be debound in air, hydrogen, oxygen, argon, nitrogen and similar gas atmospheres or in vacuum, and does not leave behind deleterious residues, either in the part or in the environment, and does so in a more controllable manner, adapted to correspond to the inorganic components to be bound and formed into the desired part, and while providing increased control of the debinding process. The debinding accelerator may be used advantageously to increase the rate of the initial debinding steps, and thereby to reduce overall production time. The tie molecule may be used to advantageously extend the completion of the debinding steps, and thereby assure that the inorganic powder retains its desired form until the onset of sintering.